The event_system manages events, handlers, resources, groups,
etc. It is now a class type, and you may invoke the operations directly
for the class. The operations are still available as functions (below).

A resource is an operation with an optional timer. The operation
describes the condition to watch for, and the timer defines the
maximum period of time for that. If the condition becomes true,
an Input_arrived, Output_readiness, or Out_of_band event
will be triggered. If the timer expires, a Timeout event will be
generated. After the event the resource remains active, and the
timeout period begins anew.

A resource is usually bound to a file descriptor. It is allowed
to watch the same descriptor for several different conditions,
but it is forbidden to watch the same descriptor for the same kind
of condition several times.

As a special case, the operation Wait is not bound to a
file descriptor, but simply starts a timer. The argument of Wait
can be used to distinguish between several timers that are active
at the same time.

Event handlers get the events one after the other, and
process them. When a handler is called for an event, there are
several possible reactions: (1) The handler can return normally,
which means that the event has been accepted, and will not be
passed to any other handler. (2) The handler can raise
Equeue.Reject, which means that the handler cannot process
the event, and that another handler should get it. (3) The handler
can raise Equeue.Terminate which means that the event has been
accepted, and that the handler is terminated (it will never be
called again). (4) The handler can raise Abort which means that
the event is deferred, and that a special abort mechanism is
triggered (see the description for Abort above), this is also
terminates the handler. The deferred event will again be processed
in the future. (5) The handler can raise any other exception.
This causes that the event is deferred, and the exception falls
through to the caller of run.

Groups are used to simplify the association of events to
handlers, and to simplify the termination of handlers (see clear).
If an event is associated with a group, only handlers associated with
the same group will get them.

There is a special Close handler which is useful to close file
descriptors no longer needed. It is called when all resources are
removed from the event system dealing with the file descriptor.
The close handler should close the descriptor. Note that close handlers
are only useful under certain circumstances.